1. Field of the Invention
The present invention relates to improvement in a nitride semiconductor laser device and a method of producing the same.
2. Description of the Background Art
From the past, prototypes of nitride semiconductor laser devices for emitting light in a wavelength range from blue to ultraviolet have been fabricated using nitride-based semiconductor materials such as GaN, InN, AlN, and a mixed crystal semiconductor thereof.
FIG. 9 is a schematic cross-sectional view exemplarily showing a main part of a conventional nitride semiconductor laser device for lasing at a wavelength of 405 nm. This semiconductor laser device includes an n-type GaN layer (thickness: 3 μm) 902; an n-type In0.05Ga0.95N buffer layer 903; an n-type Al0.05Ga0.95N clad layer (thickness: 2.0 μm) 904; an n-type GaN optical waveguide layer (thickness: 0.1 μm) 905; an In0.2Ga0.8N/n-type In0.05Ga0.95N (respective thicknesses: 4 nm/8 nm) triple-quantum-well (3MQW) active layer 906; a p-type Al0.2Ga0.8N carrier stop layer (thickness: 20 nm) 907; a p-type GaN optical waveguide layer (thickness: 0.1 μm) 908; a p-type Al0.05Ga0.95N clad layer (thickness: 0.5 μm) 909; and a p-type GaN contact layer (thickness: 0.2 μm) 910 stacked in this order on an n-type GaN substrate 901.
A ridge-like stripe 902 of 2 μm width is formed by partially etching p-type GaN contact layer 910 through to a partial depth of p-type AlGaN clad layer 909. Accordingly, the laser device of FIG. 9 has an optical confinement waveguide structure in which active layer 906 and optical waveguide layers 905 and 908 are sandwiched between clad layers 904 and 909, and then light generated in active layer 906 is confined in this waveguide structure and causes lasing action. It should be noted that FIG. 9 does not show the entire width of the laser device chip but only a part thereof in the vicinity of the ridge-like stripe.
A p-side contact electrode 911 is formed on p-type GaN contact layer 910 left after the partial etching, and insulating films 913 are formed on the partially etched areas. A p-side electrode pad 914 is formed in contact with p-side contact electrode 911. On the other hand, an n-side electrode 915 is formed on a rear surface of substrate 901.
The laser device of FIG. 9 is obtained by dividing a wafer including a large number of laser device structures into chips. To facilitate the chip division, therefore, substrate 901 is polished to have a thickness of 50 to 200 μm before n-side electrode 915 is formed thereon. Then, the nitride semiconductor laser device of FIG. 9 with a width of 300 to 400 μm is obtained by dividing the wafer into chips.
FIG. 10 is a schematic plan view of the nitride semiconductor laser device of FIG. 9 seen from above. Ridge-like stripe 912 is formed on the upper surface of the nitride semiconductor laser device, and the p-side contact electrode (not shown) is formed over the entire upper surface of the ridge-like stripe. Further, p-side electrode pad 914 is formed to cover almost the entire upper surface of the nitride semiconductor laser device.
The nitride semiconductor laser device of FIG. 10 is mounted on a stem, and thereafter wire bonding is performed in an area not including ridge-like stripe 912 on p-side electrode pad 914. Therefore, p-side electrode pad 914 is generally formed such that a circular region of more than 80 μm diameter is securely obtained for a wire bonding area.
Generally, in order to reduce the cost for a nitride semiconductor laser device, it is desirable to increase the yield rate of the laser devices obtained from one wafer by reducing the width of the laser device to 50 to 250 μm. However, when a nitride semiconductor laser device has a reduced width, problems arise in a laser device fabricated by a technique described below.
In Japanese Patent Laying-Open No. 2004-356454, trenched regions each having a stripe-like groove are formed at an interval of several hundred micrometers on a nitride semiconductor substrate in order to suppress occurrence of cracks during crystal growth of nitride semiconductor layers. Accordingly, a hill portion is naturally formed between adjacent trenched regions. By growing nitride semiconductor layers and form a layered structure thereof on a substrate subjected to such working as above (hereinafter referred to as a worked substrate), occurrence of cracks in the layered structure can be prevented, and surface flatness of the layered structure over the hill portion can be improved to some extent.
When the nitride semiconductor layers are grown to form the layered structure on the worked substrate, however, a swelling with a height of several micrometers is caused adjacent to a trenched region in an upper surface of the layered structure. Taking account of the photolithographic process in fabricating a nitride semiconductor laser device, therefore, the ridge-like stripe structure should be formed more than 90 μm away and preferably more than 110 μm away from the trenched region.
FIG. 11 is a schematic plan view showing an example of an upper surface of a bar obtained after formation of resonator end faces of each nitride semiconductor laser device (i.e., after a wafer including a large number of laser device structures is divided into bars) and just before the bar (hereinafter referred to as a laser bar) is divided into individual chips. In FIG. 11, trenched regions 1101 are formed at an interval of 800 μm for example, and then ridge-like stripes 1102 and p-side electrode pads 1103 are formed between trenched regions 1101. Every dotted line in FIG. 11 indicates a chip division plane, and the chip width is set to 200 μm for example.
As described above, in FIG. 11, a distance (L) between ridge-like stripe 1102 and trenched region 1101 should desirably be set to more than 110 μm. In order to set the chip width to 200 μm, therefore, ridge-like stripe 1102 in each of certain particular chips is set at a position different from that in the other chips, with respect to the width direction of the chip. More specifically, in FIG. 11, the position of ridge-like stripe 1102 in a laser device chip (B) is different from that in a laser device chip (A), with respect to the width direction of the chip.
The laser bar in such a state as above may cause problems as described below during the subsequent process of producing laser devices.
Firstly, after the laser bar is divided into individual laser device chips, it is necessary to inspect all the chips. In the case of using an automatic chip inspection apparatus, if the apparatus is set to determine the position of the ridge-like stripe in chip (A) as normal, then there is a problem that it determines all of chips (B) as defective.
Further, when a chip is mounted on a stem, the light-emitting point in the chip should be made coincident with the center of the stem. However, since the light-emitting point in chip (B) is different from that in chip (A) with respect to the width direction, it is necessary to mount chip (B) at a chip position changed relatively on the stem.